Grid-tied inverters are used in power supply installations, for example photovoltaic installations and wind energy installations. In the case of grid-tied inverters, a voltage or current characteristic that is output at the output of the inverter follows the corresponding characteristic in the power supply system. In the power supply installations, generators, for example photovoltaic modules in a series and/or parallel connection, produce a DC voltage that—possibly after a voltage change by a step-up converter—is supplied to a DC link circuit. Direct current from the DC link circuit is converted by the inverter into an alternating current whose frequency and voltage are suitable for supply to the power supply system. This conversion may be into single-phase or polyphase, particularly three-phase, alternating current in this case. For this, the inverter has an output bridge circuit that, depending on the number of phases of the power supply system that are intended to receive a supply, has one or more switching bridges that are usually equipped with power semiconductor switches.
The power semiconductor switches are in this case actuated according to a particular modulation pattern such that, in conjunction with filters that are arranged between the inverter and the power supply system, a preferably sinusoidal output current is produced. In the case of the pulse width modulation (PWM) methods that are frequently used, the power semiconductor switches are switched on and off at a switching frequency that is distinctly higher than the frequency of the AC voltage in a power supply system (for example a switching frequency of 3 to 30 kHz in comparison with a mains frequency of 50 or 60 Hz). Over the course of one period of the mains frequency, the ratio between switched-on time and switched-off time, called the duty ratio, is in this case altered within one switching frequency period such that a preferably sinusoidal characteristic of the output current is obtained. Known configurations for determining the duty ratios or the switching times are, by way of example, the “sine-delta modulation method”, the “space vector modulation (SVM) method” or a modified sine-delta modulation method, e.g. what is known as “third harmonic injection sine-delta modulation method”. In the case of these PWM methods, a periodic auxiliary signal or carrier signal, e.g. a triangular-wave form signal in the case of the “sine-delta modulation method”, or a clock signal in the case of the “SVM method”, is used for determining the switching times.
Even in the case of more complex modulation methods, however, the voltage produced at the output of the inverter bridges is typically not a pure sine signal, but rather exhibits e.g. frequency components at the switching frequency of the power semiconductor switches according to the modulation method.
To attain high powers, two or more inverters are frequently used in parallel, particularly in the case of larger photovoltaic installations, for example open air installations. When multiple inverters are operated in parallel, undesirable circulating or equalizing currents can arise on account of asynchronous switching times within the output bridge arrangements of the individual inverters. This occurs particularly when the periodic auxiliary signals that are used for determining the switching times of the output bridge arrangements of the individual inverters are highly phase-shifted with respect to one another. This results in asynchronous switching processes that can lead to brief voltage differences between the inverters, which in turn cause high-frequency equalizing currents. These equalizing currents can arise particularly when the inverters are not completely isolated from one another on the output voltage side. They are an undesirable additional current load for the affected electronic components, such as the AC filter capacitors, for example, which can have an adverse influence on the service life of these components.
The document US 2008/0265680 A1 describes an arrangement of multiple inverters that are directly coupled by their outputs. The inverters are controlled by PWM methods, with the auxiliary signals used therein being synchronized on the basis of a mains voltage. The effect achieved by this synchronization is that the power semiconductor switches of the coupled inverters are switched at the same times.
The document WO 2012/123559 A2 discloses a method that is suitable for inductively coupled, for example transformer-coupled, inverters. This method has provision for the auxiliary signals used for producing the actuating signals for the semiconductor power switches in the PWM method to be synchronized on the basis of the mains voltage, there being provision for a phase difference between the auxiliary signals of different inverters.
The document US 2010/0156192 A1 and the article “Voltage Control in a Battery-Operated Sinusoidal Pulse-Width-Modulated (SPWM) Photovoltaic Inverter”, Africon, 1999 IEEE, Volume 2, pages 719-724, also describe PWM methods that involve an auxiliary signal used for actuating semiconductor power switches being synchronized on the basis of the mains voltage. The synchronization is attained using a PLL (Phase Locked Loop) circuit.
On the output side, inverters usually have provision for an AC voltage filter that shapes the output-side AC signal and particularly ensures that the output current characteristic is preferably sinusoidal. For this reason, the filter is frequently also referred to as a sine filter. Effective signal shaping is possible using what is known as an LCL filter, for example, which has two inductances connected in series in a phase line and which has a capacitor arranged between the center tap between the inductances and one of the further phases, a neutral conductor or center tap of the AC bridge of the output AC signal of the inverter. Such a filter is particularly effectively suitable for attenuating the switching frequency components of the bridge voltages. However, it is material-intensive and hence costly. It has been found that particularly the inductance of such an output current filter on the power supply system side can have its inductance value reduced, or may even disappear completely if need be, if, instead of a firmly prescribed frequency of the auxiliary signal for determining the switching times of the semiconductor power switches of the output bridge arrangement, a periodic auxiliary signal having a varying frequency is used for the inverter. The reason is that when an auxiliary signal having a varying frequency is used, as outlined above, the electromagnetic interference arising on account of the pulse-modulated bridge voltage extends over a broader frequency range and hence the respective amplitudes of the electromagnetic fields turn out to be distinctly lower for a specific frequency value. Stipulations made by power supply companies for the spectral intensity of a spurious signal can then also be met using smaller inductances in the output current filter, with particularly the output-side inductance, but sometimes also the input-side inductance, being able to be reduced. If need be, the inductance on the power supply system side can even be dispensed with completely.
To produce an auxiliary signal having a variable frequency, what is known as a wobble signal is usually used, the characteristic of which modulates the frequency of the auxiliary signal. The voltage characteristic used in this case is preferably a triangular-waveform characteristic.
An inverter using a PWM method with a variable pulse frequency is known from the document DE 197 48 479 C1, for example. In the case of this method, the pulse frequency is dependent on the characteristic of the alternating current produced, the pulse frequency being higher by a multiple at zero crossing of the alternating current than in the region of the maximum amplitude of the alternating current. This method minimizes switching losses for power semiconductor switches of the inverter and at the same time minimizes harmonics for the current characteristic of the alternating current produced.
Further purpose of an auxiliary signal having not a constant but rather a variable frequency is to allow a particularly prescribed and desired frequency of an auxiliary signal to be adjusted even when the frequency resolution of the signal generator that produces the auxiliary signal is not adequate for adjusting the desired frequency. In this case, it is possible to hop to and fro between two adjustable frequencies of a periodic auxiliary signal at a particular prescribed duty ratio such that the desired frequency is obtained at least on average. The aforementioned wobble signal for the auxiliary frequency is a square-wave signal in this case.
When two or more inverters that use a wobbled, i.e. varying-frequency, auxiliary voltage signal are interconnected, it is not possible to use the method outlined above for synchronizing the periodic auxiliary signals to the mains frequency. Hence, the known methods cannot be used to suppress the equalizing currents.